Remove duplicate test fixture files

These are not test files:
- st2actions/tests/unit/test_async_runner.py
- st2actions/tests/unit/test_polling_async_runner.py

It looks like they were copied to st2tests/st2tests/mocks/runners/
at some point. Nothing imports from or uses the copies in st2actions,
so just delete them.

Author: cognifloyd